A passive sonar furthermore comprises a supply unit, making it possible to produce energy intended to supply the sensors and a processing unit intended to process the measurements arising from the various sensors so as to detect and then optionally identify and locate objects. The supply unit and the processing unit are sited remotely aboard the marine vessel or on a land station. Provision is made for a link between the acoustic antenna and the supply unit as well as the processing unit. When this link is a link comprising solely one or more optical fibers, one speaks of an all-optical acoustic antenna. Stated otherwise, the links used for the transmission of the supply energy from the supply unit to the antenna and for the transmission, to the processing unit, of the information arising from the sensors are optical fibers. These solutions are of small bulk, are cheap and lightweight. They are insensitive to electromagnetic disturbances. They make it possible, on account of the weak energy attenuation in the optical fibers, to read the measurements performed by the sensors at several kilometers, and to do so without any electrical energy source being connected to the sensors.
All-optical acoustic antennas comprise a plurality of fiber optic hydrophones acting on respective sensitive zones of one or more optical fibers. By fiber optic hydrophone is meant a hydrophone comprising an optical fiber which delivers a measurement signal which is an optical signal. This optical signal is conveyed by an optical fiber. A measurable physical property of this optical signal is representative of the acoustic pressure to which the hydrophone is subjected.
Hydrophones are acoustic sensors intended to measure an acoustic pressure also called dynamic pressure, that is to say fast variations of low pressure. The acoustic pressure measured by a hydrophone is typically between 30 and 200 dB reference 1 μPa and varies at frequencies lying between 1 Hz and 100 kHz.
When acoustic sensors are submerged in water they are also subjected to hydrostatic pressure also called static pressure which increases by approximately 10 000 Pa per meter of water. Hydrophones being used at depth, the hydrostatic pressure undergone by a hydrophone is typically greater than 10 000 Pa. In order to make it possible to measure the acoustic pressure, hydrophones filter the hydrostatic pressure, for example by means of an integrated mechanical or bolt-on high-pass filter of the electrical type. Stated otherwise, in order to avoid saturation phenomena and allow the measurement of the acoustic pressure, acoustic sensors do not make it possible to measure the hydrostatic pressure, that is to say a pressure at least equal to 10 000 Pa, otherwise they would not make it possible to measure the acoustic pressure, the difference in amplitude between the acoustic pressure and the static pressure being of the order of 100 000 000 to 1 000 000 000 Pa.
The aim of passive sonars is to detect and to locate objects in the water. This aim is achieved by means of the acoustic reception antenna but the location of objects makes it necessary to integrate into the acoustic antenna, one or more additional non-acoustic sensors such as, for example, at least one heading sensor, at least one accelerometer, at least one temperature sensor and/or at least one static (or immersion) pressure sensor. The measurements arising from these sensors are used for the detection and/or the location of objects in the water. By non-acoustic sensor is meant a sensor configured to measure one or more physical quantities, each measured physical quantity being a measurement other than an acoustic pressure.
Some of these sensors exist in the form of optical fiber sensors. Such sensors deliver an optical signal representative of the quantity to be measured, said optical signal being conveyed by an optical fiber. There exist for example optical fiber temperature sensors and optical fiber static-pressure sensors. Their integration into the streamer is then easy. However, certain non-acoustic sensors, such as for example heading sensors, exist only in the form of sensors with electrical output. By sensor with electrical output is meant a sensor whose sensitive element delivers an electrical signal, such as for example a voltage or an intensity, representative of a measured physical quantity. The integration of this type of sensor into an all-optical measurement system and in particular its integration into an acoustic antenna of streamer type is then complex.